Controlled pressure consumable electrode vacuum arc remelting process

ABSTRACT

Manganese or other high vapor pressure elements are prevented from volatilizing during vacuum arc remelting by introducing a small flow of inert gas into the mold and varying the rate of exhaust of gases from the mold from a maximum value when the arc zone is at the bottom of the mold to a minimum value when the arc zone is at the top of the mold so as to maintain a pressure in the arc zone higher than the vapor pressure of the volatile element.

United States Patent 1 1 Schlatter 1451 May 28, 1974 CONTROLLED PRESSURE CONSUMABLE ELECTRODE VACUUM ARC REMELTING PROCESS [75] Inventor: Rene Schlatter, Derry Township,

Westmoreland County, Pa.

[73] Assignee: Latrobe Steel Company, Latrobe,

[22] Filed: Mar. 28, 1973 [2,1] Appl. No.: 345,670

Manganese or other highvapor pressure elements are prevented from volatilizing during vacuum arc remelting by introducing a small flow of inert gas into the mold and varying the rate of exhaust of gases from the mold from a maximum value when the arc zone is at the bottom of the mold to a minimum value when the arc zone is at the top of the mold so as to maintain a pressure in the arc zone higher than the vapor pressure of the volatile element.

10 Claims, 2 Drawing Figures [52] U.S. Cl. 164/52, 164/68 [51] Int. Cl 822d 27/02 [58] Field of Search 164/52, 252, 68, 259

[56] References Cited UNITED STATES PATENTS 2,952,723 9/1960 Garmy 164/259 x 3,072,982 1/1963 Gordon et a1 164/68 X g 400 PRESSURE PROFILE LLI MELTING CURRENT, KA

CURRENT PROGRAM TIME Hows IATENIEIIIAYZB IsII 3.812899 SHEET 2 BF 2 MANGANESE DISTRIBUTION PRIOR ART THIS INVENTION TOP 0.67%Mn 1 1 1/ 1 O.8I% Mn 0.75% Mn I I I I o.s4% Mn 0.79% Mn 1 O.82%Mn 0.85% Mn 1 I I I -O.84% Mn BOTTOM Fig.2.

CONTROLLED PRESSURE CONSUMABLE ELECTRODE VACUUM ARC REMELTING PROCESS This invention relates to the consumable-electrode vacuum arc remelting process of making alloys. It is more particularly concerned with such a process which is adapted to the melting of alloys containing volatile constituents.

In the consumable-electrode vacuum arc melting process an electrode cast from the alloy desired is remelted in an ingot mold by drawing an are between the lower end of the electrode inserted into the mold and the remelted alloy in the bottom of the mold. The mold is fully enclosed and connected to a vacuum pump,

which maintains the pressure therein at a desired low value. Some of the undesired constituents in the cast electrode are volatilized in the arc and drawn off by the vacuum system. This process as above described is rather wasteful in the production of alloys which contain relatively high-vapor-pressure alloying elements, as these later are also volatilized and lost in varying amounts. Examples of such elements and their pertinent properties are listed in the table.

Physical Properties of Various Volatile Elements Vapor Pressure in Melting Boiling Torr (mmI-Ig) at Point C Point "C l500C Mn I245 2l50 2 l0' (Tu 1083 2595 3X 10" N2 2 l (l I96 Mg 650 H07 10 Ca 838 1440 8 X l Ba 7 l4 I640 X 10" Se 217 685 10 Te 450 990 10 It is an object of my invention to provide a consumable-electrode vacuum remelting process in which the loss of volatile alloying elements is minimized. It is another object to provide such a process in which the content of volatile alloying element is substantially uniform throughout the ingot. Other objects of my invention will become apparent in the course of the description thereof which follows.

FIG. 1 is a composite graph showing the arc current and the pressure at the top of the mold both measured against time.

FIG. 2 shows the manganese distribution in both the prior art ingot and in an ingot made according to this invention.

I have found that variations in the volatilization of volatile elements are occasioned by the geometry of the consumable are furnace. It consists of a narrow upright cylindrical ingot mold, the upper end of which is connected to the vacuum producing apparatus. The consumable-electrode is an elongated cylinder which initially extends the full length of the mold and is spaced therefrom by a narrow annular gap. A 10 inch diameter electrode is generally used in a 12 inch diameter mold, for example, and a 28 inch electrode in a 30 inch diameter mold. The mold is exhausted from the top and the gases drawn from the bottom and intermediate levels must travel through this narrow gap. I have discovered that the pressure at the bottom of the mold is considerably greater than that at the top because of the resistance offered by the gap to flow of gases therethrough, and that the pressures at intermediate levels have values between the extremes. The amount of alloy volatilized varies inversely with the pressure over the molten alloy, and accordingly is less at the bottom of the mold than at the top.

In my process, therefore, I bleed into the continuously evacuated system an inert gas at a low rate which is essentially constant from the commencement of melting to the end of refining when the remelted alloy fills the ingot mold. The rate of inert gas flow is generally maintained between 5 and 100 cubic feet per hour, depending upon the size of the ingot melted. Inert gases which are suitable are argon, nitrogen, helium, neon,

carbon monoxide and hydrogen. Those of higher molecular weight are preferred. The pumping speed of the vacuum system is adjusted in accordance with the progress of the remelting so that the pressure above the molten alloy pool in the mold is maintained at a value sufficient to suppress the volatilization of volatile alloying elements to tolerable limits. This pressure may be substantially constant.

In a continuously pumped system the pressure at the head of the ingot mold is continuously measured. I do not know of any feasible way of measuring during melting the pressure over the molten alloy pool when it is below the top of the mold. The pressure at the bottom of the mold, however, may be ten times or, so that at the head of the mold. The value of the multiple depends, of course, on the size of mold and electrode, among other things. A first approximation, therefore, of my process comprises varying the throughput of the gases given off by the melt and of the inert gas bled into the continuously pumped system as the remelting proceeds, uniformly from a higher value at the start of refining to a lower value at the finish of remelting. The variation is conveniently accomplished manually by adjusting the flow of the gas as by a throttling valve from time to time, so that the pressure increases stepwise with time. The variation is conveniently accomplished automatically, if desired, by continusouly adjusting the valve opening so that the pressure increases smoothly as the melt proceeds. Manganese-containing ingots, for example, remelted under the conditions above described, have manganese contents which do not vary greatly from top to bottom. I

I have found, however, that uniform increase in pressure from bottom to top of the ingot does not provide optimum uniformity of volatile element content in the ingot. The pressure gradient in the mold is, in fact, not uniform from bottomto top, but decreases at a greater rate from a level approximately midway of the mold to 3 the top than it does from the bottom to this intermediate level. I have found that the increase in pressure required near the top of the mold can be correlated, to a degree, with the arc current or melt rate, respectively, which, of course, is continuously measured. The pressure above the melt must be increased as the melting current is decreased because the dwell time of the mo]- ten alloy increases, allowing for more volatilization. That correlation is shown graphically in the attached FIG. 1. The lower curve is the arc current plotted against time while the upper curve is the pressure measured at the top of the mold, likewise plotted against time.

At the commencement of remelting the arc current rises rapidly from zero to a high value which is maintained for a short time and is then caused to decrease linearly for a period of time comprising about half of the aggregate heat time. The current is initially adjusted to a high value to speed up the heating of the bottom of the mold where heat losses are greatest. As the mold heats up and the ingot grows within it the arc current is cut back gradually over the time period previously mentioned to its steady-state value for the ingot size in question. It is thenmaintained at this value until near the end of the remelt, when it is sharply reduced as is shown. During the time the arc current is decreasing the pressure at the head of the mold, in my preferred process, is increased gradually at a relatively low rate. When the arc current levels off the pressure is caused to increase at a higher rate, approximately twice that of its previous increase, until the arc current is reduced near the end of the remelt. From this point until the end of the heat, which is a small portion of the total heat time, the shape of the pressure curve is roughly the inverse of the arc current curve. These final adjustments of the pressure are made to minimize excessive losses of volatile alloying elements during the hot topping operation.

ln my process the gas flow to the vacuum system is adjusted to maintain a pressure in the arc zone which is above the vapor pressure of the volatile element but within an acceptable pressure range for vacuum are remelting. Those skilled in that art know that this pressure range is between about 05 Torr and 20 ,Torr. Melting is carried out preferably at pressures below about 5 Torr. if the pressure exceeds the upper figure here mentioned, are instability is encountered. When the volatile element is one having a vapor pressure at its melting point well within this range it is sufficient to maintain the pressure in the arc zone above that vapor pressure, but it can vary up to pressures short of arc instability without detrimental effect on the homogeneity ofthe ingot.

FIG. 2 attached is a schematic elevation of a 22,000 pound vacuum arc remelted ingot of low alloy steel, showing on the left side the manganese content at four locations over the ingot length of an ingot remelted by the practice of the prior art, and on the right side the manganese content of a like ingot remelted. in accordance with my preferred process here described. it is evident that at the bottom of the ingot the manganese content is the same by either process, about 0.85 percent. At a level about one-third of the way up the manganese content in the process of the prior art is 0.79 percent, whereas in my process it is 0.82 percent. At a level about two-thirds of the way from the bottom the manganese content in the process of the prior art is top of the ingot. the manganese content resulting from the prior art process is 0.67 percent, and from my process 0.81 percent. Thus, the overall manganese content variation in prior art remelting is 0.18 percent, or about 21 percent of the bottom manganese content, but in my process only 0.03 percent, or about 3.5 percent of the bottom manganese content. The ingot in addition to manganese contained 0.4 percent carbon, 1.6 percent silicon, 0.005 percent phosphorus. 0.008 percent sulfur, 1.8 percent nickel, 0.4 percent molybdenum, and 0.8 percent chromium.

While it is satisfactory in this process to introduce the inert gas into the top of the mold, it is also satisfactory to introduce the inert gas in the arc zone through a tube or pipe which is welded to the electrode.

in the foregoing specification l have described a presently preferred embodiment of this invention, however, it will be understood that this invention can be otherwise embodied within the scope of the following claims.

I claim:

1. in the process of consumable-electrode continuously exhausted vacuum arc remelting in an ingot mold of an alloy containing a desired constituent which volatilizes in the arc zone, the improvement comprising introducing a small flow of inert gas into the ingot mold and varying the rate of exhaustof gases from the mold from a maximum value when the arc zone is at the bottom of the mold to a minimum value when the arc zone is at the top of the mold so as to maintain a pressure in the arc zone higher than the vapor-pressure of the volatile constituent.

2. The process of claim 1 in which the pressure in the arc zone is a substantially constant pressure.

3. The process of claim 1 in which the pressure in the arc zone is maintained below that pressure at which are instability occurs.

4. The process of claim 1 in which the pressure in the arc zone is maintained below about 20 Torr.

5. The process of claim 1 in which the pressure in the arc zone is maintained below about 5 Torr.

6. The process of claim 1 inwhich the flow of inert gas is introduced into the ingot mold at the level of the arc zone.

7. The process of claim 1 in which the flow rate of inert gas into the mold is between about 5 and about cubic feet of gas per hour, the lower rates being used with smaller ingot molds and the higher rates with larger ingot molds.

8. The process of claim 1 in which the rate of exhaust is adjusted to provide a pressure at the top of the mold which increases from the beginning of remelting to. the end thereof.

9. The process of claim 8 in which the arc current is maintained at a steady value over a latter portion of the remelting period and in which the rate of exhaust is adjusted so that the rate of increase in pressure at top of the mold during that latter portion is about twice the rate of increase of that pressure during the portion of the remelting period immediately preceeding that latter portion. a

10. The process of claim 9 in which the arc current is gradually reduced during the portion of the remelting period immediately preceeding that latter portion from its maximum value at the commencement of remelting, and in which the commencement of the latter portion is the approximate midpoint of the remelting period. 

2. The process of claim 1 in which the pressure in the arc zone is a substantially constant pressure.
 3. The process of claim 1 in which the pressure in the arc zone is maintained below that pressure at which arc instability occurs.
 4. The process of claim 1 in which the pressure in the arc zone is maintained below about 20 Torr.
 5. The process of claim 1 in which the pressure in the arc zone is maintained below about 5 Torr.
 6. The process of claim 1 in which the flow of inert gas is introduced into the ingot mold at the level of the arc zone.
 7. The process of claim 1 in which the flow rate of inert gas into the mold is between about 5 and about 100 cubic feet of gas per hour, the lower rates being used with smaller ingot molds and the higher rates with larger ingot molds.
 8. The process of claim 1 in which the rate of exhaust is adjusted to provide a pressure at the top of the mold which increases from the beginning of remelting to the end thereof.
 9. The process of claim 8 in which the arc current is maintained at a steady value over a latter portion of the remelting period and in which the rate of exhaust is adjusted so that the rate of increase in pressure at top of the mold during that latter portion is about twice the rate of increase of that pressure during the portion of the remelting period immediately preceeding that latter portion.
 10. The process of claim 9 in which the arc current is gradually reduced during the portion of the remelting period immediately preceeding that latter portion from its maximum value at the commencement of remelting, and in which the commencement of the latter portion is the approximate midpoint of the remelting period. 